HD Extreme 3 input

Hi,
can anyone tell me, how can I choose between input ports?
I have one SDI input signal on the first (A) port, and an another one on the secondary (B) port. When I capture, I default see the port A signal, but I want to change it to B during capture. Or is it impossible?
Can I set up the card to automaticly switch to the port B if there are no signal on A any more?